[patch 11/15] mm/filemap: don't allow partially uptodate page for pipes

[patch 11/15] mm/filemap: don't allow partially uptodate page for pipes

[akpm]

From: Eryu Guan <guaneryu@gmail.com>
Subject: mm/filemap: don't allow partially uptodate page for pipes

Starting from 4.9-rc1 kernel, I started noticing some test failures of
sendfile(2) and splice(2) (sendfile0N and splice01 from LTP) when testing
on sub-page block size filesystems (tested both XFS and ext4), these
syscalls start to return EIO in the tests.  e.g.

sendfile02    1  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 26, got: -1
sendfile02    2  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 24, got: -1
sendfile02    3  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 22, got: -1
sendfile02    4  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 20, got: -1

This is because that in sub-page block size cases, we don't need the whole
page to be uptodate, only the part we care about is uptodate is OK (if fs
has ->is_partially_uptodate defined).  But page_cache_pipe_buf_confirm()
doesn't have the ability to check the partially-uptodate case, it needs
the whole page to be uptodate.  So it returns EIO in this case.

This is a regression introduced by commit 82c156f85384 ("switch
generic_file_splice_read() to use of ->read_iter()").  Prior to the
change, generic_file_splice_read() doesn't allow partially-uptodate page
either, so it worked fine.

Fix it by skipping the partially-uptodate check if we're working on a pipe
in do_generic_file_read(), so we read the whole page from disk as long as
the page is not uptodate.

I think the other way to fix it is to add the ability to check & allow
partially-uptodate page to page_cache_pipe_buf_confirm(), but that is much
harder to do and seems gain little.

Link: http://lkml.kernel.org/r/1477986187-12717-1-git-send-email-guaneryu@gmail.com
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/filemap.c |    3 +++
 1 file changed, 3 insertions(+)

diff -puN mm/filemap.c~mm-filemap-dont-allow-partially-uptodate-page-for-pipes mm/filemap.c
--- a/mm/filemap.c~mm-filemap-dont-allow-partially-uptodate-page-for-pipes
+++ a/mm/filemap.c
@@ -1732,6 +1732,9 @@ find_page:
 			if (inode->i_blkbits == PAGE_SHIFT ||
 					!mapping->a_ops->is_partially_uptodate)
 				goto page_not_up_to_date;
+			/* pipes can't handle partially uptodate pages */
+			if (unlikely(iter->type & ITER_PIPE))
+				goto page_not_up_to_date;
 			if (!trylock_page(page))
 				goto page_not_up_to_date;
 			/* Did it get truncated before we got the lock? */
_